1. Field of the Invention
The invention relates to a method of converting a direct current voltage of a source of direct current voltage, more specifically of a photovoltaic source of direct current voltage, into an alternating current voltage at a frequency through a bridge circuit comprising switching elements and free-wheeling elements.
2. Description of the Prior Art
The document DE 197 32 218 C1 describes a transformerless inverter circuit arrangement with an inverting buck-boost converter circuit and a non-inverting converter circuit. Such type inverters are used for coupling photovoltaic systems. The circuit comprises a direct connection that is steady in terms of potential, that is, a conductor connection that remains at a fixed potential. At need, it is possible to use this direction connection as the neutral of the system between one of the two direct current connection terminals and one of the alternating current connection terminals and to couple thereto the negative terminal of the source of direct current for example, which has the great advantage of avoiding EMC problems. The circuit is intended to provide a lightweight transformerless inverter of a small construction that also ensures high personal safety and causes little EMC problems.
An inverter circuit having a semiconductor bridge, a solar generator and a switching element mounted between the solar generator and the semiconductor bridge is known from DE 103 12 921 A1. This circuit arrangement is configured such that the switching element opens when a defined direct current voltage of the solar generator is exceeded and the switching element is switched on when the voltage remains below the generator voltage so that the input voltage range, which is possible for the inverter circuit or the consumer, is increased. The circuit as configured does not avoid high-frequency voltages and does not eliminate EMC problems.
The document DE 102 21 592 A1 discloses a transformerless inverter with a bridge circuit. In this circuit, there are provided two separate electrical connection paths in each of which there are provided one switch and rectifier diodes connected in series. They are connected in the forward direction in the various connection paths. As contrasted with symmetric clocking, this circuit permits to clearly lower a current ripple in the output current. The additional free-wheeling diodes allow for ohmic decoupling between a solar generator and the alternating current voltage connection terminals. This permits to avoid high-frequency voltage transients at the connecting lines of the generator and improves EMC behaviour as a result thereof.
It is moreover known to utilize for transformerless photovoltaic inverters an H-bridge circuit with fourth semiconductor switches that are alternately connected in such a manner that, from the direct current voltage applied, an alternating current voltage is obtained between the branch connections of the bridge halves. What is more, the switching elements are provided with symmetric clocking. An upper switch of a bridge half is hereby clocked and pulse-width modulated at a high clock rate in synchronism with another lower switch of the other bridge half.
To reduce harmonics of the thus produced alternating current voltage, reactors are utilized. In order to keep harmonics in the alternating current voltage low, the reactors must be of quite large dimensions. This solution results in quite high hysteresis losses in the reactors so that the efficiency of the circuit is reduced.
Further losses also occur because two switching elements are switched on and off concurrently and because, at free-wheeling state, the current flows into the direct current voltage intermediate circuit through two free-wheeling diodes. At free-wheeling state, the direct current voltage in the intermediate circuit acts as a reverse voltage, which results in an increased current ripple and in an increased power loss.
In order to reduce these losses, it is known to asymmetrically clock the bridge. This means that while the upper switches are gated with mains frequency, the lower switches are gated at the high clock rate. As a result, at free-wheeling state, the reverse voltage from the intermediate circuit is eliminated as the current only commutates through a diode and a switch. This results in a lower current ripple and in reduced losses. This asymmetric gating however generates high-frequency potential fluctuations at the terminals of the photovoltaic generator which worsen the EMC behaviour of the generator.
A provision preventing the drawbacks of the two solutions is shown and described in the printed document DE 102 21 592 A1. There is hereby provided that two connection paths are additionally provided between the outputs of a bridge circuit or of an H-bridge. Four semiconductor components are located in the connection paths, namely each being another switching element with an associated driver stage and a diode connected in series.
Accordingly, the two drawbacks are avoided at the price of a more complex circuitry due to the much greater number of components which translates into a less reliable circuit and in increased material cost.